neutral subatomic particle that is a constituent of every atomic nucleus except ordinary hydrogen. It has no electric charge and a rest mass equal to 1.67495 × 10⁻²⁷ kg—marginally greater than that of the proton but nearly 1,840 times greater than that of the electron. Neutrons and protons, commonly called nucleons (nucleon), are bound together in the dense inner core of an atom, the nucleus, where they account for 99.9 percent of the atom's mass. Developments in high-energy particle physics (physics) in the 20th century revealed that neither the neutron nor the proton is a true elementary particle (subatomic particle); rather, they are composites of extremely small elementary particles called quarks (quark). The nucleus is bound together by the residual effect of the strong force, a fundamental interaction that governs the behaviour of the quarks that make up the individual protons and neutrons.

The neutron was discovered in 1932 by the English physicist James Chadwick (Chadwick, Sir James). Within a few years after this discovery, many investigators throughout the world were studying the properties and interactions of the particle. It was found that various elements, when bombarded by neutrons, undergo fission (binary fission)—a type of nuclear reaction that occurs when the nucleus of a heavy element is split into two nearly equal smaller fragments. During this reaction each fissioned nucleus gives off additional free neutrons, as well as those bound to the fission fragments. In 1942 a group of American researchers, under the leadership of the physicist Enrico Fermi (Fermi, Enrico), demonstrated that enough free neutrons are produced during the fission process to sustain a chain reaction. This development led to the construction of the atomic bomb. Subsequent technological breakthroughs resulted in the large-scale production of electric power from nuclear energy. The absorption of neutrons by nuclei exposed to the high neutron intensities available in nuclear reactors has also made it possible to produce large quantities of radioactive isotopes (radioactive isotope) useful for a wide variety of purposes. Furthermore, the neutron has become an important tool in pure research. Knowledge of its properties and structure is essential to an understanding of the structure of matter in general. Nuclear reactions induced by neutrons are valuable sources of information about the atomic nucleus and the force that binds it together.

A free neutron—one that is not incorporated into a nucleus—is subject to radioactive decay (radioactivity) of a type called beta decay. It breaks down into a proton, an electron, and an antineutrino (the antimatter counterpart of the neutrino, a particle with no charge and little or no mass); the half-life for this decay process is 614 seconds. Because it readily disintegrates in this manner, the neutron does not exist in nature in its free state, except among other highly energetic particles in cosmic rays (cosmic ray). Since free neutrons are electrically neutral, they pass unhindered through the electrical fields within atoms and so constitute a penetrating form of radiation, interacting with matter almost exclusively through relatively rare collisions with atomic nuclei.

Neutrons and protons are classified as hadrons (hadron), subatomic particles that are subject to the strong force. Hadrons, in turn, have been shown to possess internal structure in the form of quarks, fractionally charged subatomic particles that are thought to be among the fundamental components of matter. Like the proton and other baryon particles, the neutron consists of three quarks; in fact, the neutron possesses a magnetic dipole moment (magnetic dipole)—i.e., it behaves like a minute magnet in ways that suggest that it is an entity of moving electric charges.